The present invention relates to an averaging method for elimination of periodic strays, e.g. stray signals from a main signal source and a circuit arrangement in a system measuring evoked responses for applying the method.
The averaging method for elimination of periodic strays, e.g. stray signals from a main signal source is suitable for use in analog-digital computers employing an averaging procedure. The averaging procedure elicits evoked responses from a set of signals U.sub.zs recorded from an examined subject. The set of signals U.sub.zs comprises: internal noise U.sub.szw ; individual evoked responses Uow from the examined subject; and periodic strays U.sub.zp induce in this subject from an environment comprising a source of the mentioned strays, the specified components of the set of signals being independent.
The above statements on the set of signals U.sub.zs can be presented by the following formula: U.sub.zs =U.sub.szw +U.sub.ow +U.sub.zp. The set of signals U.sub.zs does not comprise the component of apparatus noise U.sub.sza resulting from the operation of the apparatus which picks up and amplifies the set of signals U.sub.zs, because the apparatus internal noise should be so small as to be neglected within the set of signals U.sub.zs during its continuous monitoring e.g. when observing this set on a monitor screen of an averaging computer. This noise can be recognised as the characteristic white noise, which due to its properties is non-synchronous with the aforementioned signals U.sub.zs.
The mathematical basis of the averaging procedure has been described in a publication by J. S. Bendat: "Mathematical Analysis of Average Response Values for Non-stationary Data", IEEE Transactions on Bio-Medical Engineering, BME-11: 72-81, 1964, and its technicalclinical realization has been described by J. Kopec: "Polish Computer ANOPS for Medical Research and its Clinical Application", Acta Physiologica Polonica, 21: 113-123, 1970.
Hitherto both the internal noise U.sub.szw of the examined subject and the periodic strays U.sub.zp were considered to be non-synchronous signals in relation to the individual evoked responses U.sub.ow and the averaging procedure could be described by the following formula: ##EQU1## where n=1, 2, 3, . . . , N; N at least several hundred.
As an example of the above-mentioned situation, with 256 sweeps of the averaging procedure, the square root of N equals sixteen, and with maximum amplitudes of the internal noise U.sub.szw and of the periodic strays U.sub.zp equal to 64 .mu.V then their reduction as a result of the averaging procedure is sixteen-fold, thus their maximum averaged amplitudes U.sub.szw and U.sub.zp are not greater than 4 .mu.V.
According to the above, the averaging procedure, from the practical viewpoint, efficiently reduces the noise and the strays when the ratio of the maximum amplitudes of the noise and strays to the least significant amplitudes of the individual evoked responses is less than 20:1 and the reduction is proportional to the square root of the number of sweeps of the averaging procedure provided that the noise and strays are non-synchronus in relation to the individual evoked responses.
In unfavourable measuring conditions, even if the requirements regarding the said ratio are fulfilled, amplitudes of the periodic strays U.sub.zp may become comparable with amplitudes of the individual evoked responses U.sub.ow and the strays may become synchronous in relation to the individual evoked responses U.sub.ow, so that the averaged but nevertheless significant periodic strays U.sub.zp can be superimposed on the averaged evoked response U.sub.ow. The superimposed signal may distort the averaged response to such an extent that its waveform is difficult to interpret.
The above-described situation can be described by the following formula: ##EQU2## where n=1, 2, 3, . . . , N; N at least several hundred.
As an example of the above-described situation, with 256 sweeps of the averaging procedure the square root of N equals sixteen and where the maximum amplitudes of the internal noise U.sub.szw and of periodic strays U.sub.zp are equal to 64 .mu.V then the sixteen-fold reduction in the averaging procedure is effective only in respect of the averaged internal noise U.sub.szw resulting in its amplitude not exceeding 4 .mu.V, while the averaged periodic strays U.sub.zp behave as the averaged evoked response U.sub.ow, are not reduced, and remain equal to 64 .mu.V. In such a case, other known means for reduction of noise and various strays are commonly used, i.e, analog and/or digital filtering means. The known filter circuits may be divided into: low-pass filters--eliminating strays and higher frequency signals, high-pass filters--eliminating strays and lower frequency signals and band-stop filters--eliminating strays and signals of frequency equal to strays frequency. Each kind of the above-mentioned filters interfers with and distorts the original waveform of the signals U.sub.zs. Interference and distortions introduced by the said filters cause elimination not only of strays but also signals of component frequencies comprised in the evoked responses, as the responses usally comprise signals of frequencies comparable to the fundamental frequency of the strays.
The evoked response measuring sut-ups, known to us, described and manufactured by such companies as: NICOLET BIOMEDICAL CO.--USA, MEDELEC LIMITED--Great Britain, DISA ELEKTONIK A/S--Denmark, TECHNICAL UNIVERSITY OF WARSAW--Poland are only able to eliminate periodic strays by means of the averaging procedure, and in the case where the periodic strays U.sub.zp and the individual evoked responses U.sub.ow are synchronous, can be supported by the known filtering circuits.